1. Field of the Invention
The invention relates to a method for the measurement of the oxygen partial pressure or the oxygen content in a measurement gas in at least one measurement cycle using a solid electrolyte cell having at least one oxygen-conducting solid electrolyte, and having at least one reference electrode as well as at least one measurement electrode. The at least one measurement electrode is in communication with the measurement gas and the at least one reference electrode is in communication with a reference gas or a reference volume separated from the measurement gas. A current is imposed via the electrodes on the solid electrolyte cell for pump operation and a measurement voltage (UM) is tapped at the electrodes.
2. Description of the Related Art
Methods and devices or sensors for measuring the oxygen content of a measurement gas are known in various designs. Oxygen sensors are known, for example, which operate according to a diffusion-limiting amperometric method. However, these oxygen sensors are only suitable to a certain extent for measurement at high air humidity, since as a result of the design of the sensor, moisture can penetrate into the interior of the respective oxygen sensor via diffusion openings and can lead to problems there. In particular, condensed moisture can lead to destruction of such a sensor upon starting up again.
Oxygen sensors are further known which operate according to a potentiometric method (Nernst cell). These sensors are predominantly used for measurement of small oxygen concentration in the area of exhaust gas monitoring. Due to the avoidance of diffusion openings, these sensors are more robust with respect to moisture. In order to determine the oxygen partial pressure or the oxygen content, the oxygen partial pressure of a reference gas or a reference volume is compared with the oxygen partial pressure of the measurement gas. More specifically, using a solid electrolyte cell in the simplest case includes at least one first electrode in the area of the reference gas, a second electrode in the area of the measurement gas and at least one oxygen-conducting solid electrolyte, for example, made of zirconium dioxide (zirconium IV oxide) between the first electrode and the second electrode. The measurement voltage applied to the electrodes determines, according to the so-called Nernst equation, the oxygen partial pressure quotient between the reference gas and the measurement gas. This simplified potentiometric method however assumes that the oxygen partial pressure in the reference volume is constant, i.e. the reference chamber accommodating the reference gas or reference volume is absolutely tightly sealed which in practice is not attainable or at best only with an economically unjustifiable expenditure.
In order to avoid this disadvantage, it is usual that the oxygen partial pressure in the reference volume is re-set at periodic intervals. Specifically, the reference volume is re-set using an additional solid electrolyte cell, which is located between the reference volume and the measurement gas, and the solid electrolyte cell is operated as a pure pump cell. However, this also means an additional, not inconsiderable, construction expenditure. The combination of the functions pumping and measuring in a single solid electrolyte cell has hitherto failed inter alia in that the voltage measured at the electrodes of such a solid electrolyte cell is additionally falsified by the voltage drop produced inside the solid electrolyte cell by the pump current, which is substantially dependent on the internal resistance of the solid electrolyte and is difficult to eliminate since the internal resistance of the solid electrolyte varies significantly with the temperature and lifetime thereof.
In particular, a method (EP 0 698 209) is also known in which an attempt is made to eliminate the disturbance variable produced by the voltage drop of the pump current whereby a solid electrolyte cell is alternately operated as pump and measurement cell. In this alternating mode, however polarisation effects occur which also falsify the measurement result.
In another known method (DE 198 00 027) it is proposed to reduce these polarisation effects which falsify the measurement result by a short current pulse having inverted polarity.